1. Field of the Invention
This invention relates to the electrolysis of sodium chloride brines and more specifically to decreasing the rate of mercury butter formation in a mercury electrolytic cell during the electrolysis therein of a sodium chloride brine containing strontium by processes wherein the strontium content is lowered to 0.5 ppm or less prior to electrolysis.
2. Description of the Prior Art
The production of chlorine by the electrolytic decomposition of sodium chloride brines is well known in the art. When such an electrolysis is performed in a mercury electrolytic cell, a thick flowing or solid material, termed "mercury butter", is generally formed on the surface of the flowing mercury cathode. This mercury butter is composed chiefly of mercury with small amounts of metallic impurities, mainly iron. The presence of the mercury butter in the electrolytic cell is undesirable in that it effects a decrease in current efficiency of the cell and increases the hydrogen content of the chlorine produced by the cell, thereby presenting a safety hazard and reducing the amount of chlorine that can be subsequently liquefied from the gases so produced. In addition, the presence of mercury butter in the cell requires purification of the mercury which becomes more complex and costly as the amount of mercury butter formed increases and presents a hygiene problem.
It has been the practice to purify sodium chloride brines prior to electrolysis in a mercury electrolytic cell so as to eliminate undesirable impurities present in the brine. A typical purification process is disclosed in U.S. Pat. No. 2,746,472 (issued in 1956 to Cady et al.). This purification process, however, produces brines containing strontium which, when electrolyzed in a mercury electrolytic cell effects the formation of an undesirable amount of mercury butter.